The main point of difference between land prices in Blue Mars and Second Life is that in Blue Mars the land size is user-defined (from 128m x 128m up to 16km x 16km, but see a note on this below), and you are essentially paying for concurrent user capacity, which gets cheaper the more you buy, while in Second Life you are paying for a fixed land size (256m x 256m) and 15,000 prims.

Blue Mars
Blue Mars has 4 standard packages, (but other packages may be negotiated as needed). These are:

For each package the land size is user-defined, and can be anything from 128m x 128m up to 16km x 16km. In practice I have found the best size is 2km x 2km, as the resolution of terrain textures are optimum at that size, while the higher sizes tend to suffer from instability on low-spec machines.

Although not strictly part of the land cost, it should also be noted that where land is used for commercial purposes (stores, etc) Avatar reality takes 25% of all transactions, with the land owner retaining 75%.

While Avatar Reality does not use the concept of prim limits in their land prices, there is a physical compressed file size limit of between 250MB and 500MB for all City Data, which is currently extended to 1GB until February 1st, 2010.

Second Life
Second Life has two pricing structures for virtual land, determined by whether the land is 'mainland region', i.e. it is part of a larger continent, so you may have neighbours on all four sides of your region, or 'private island region', where your region has no neighbours on any side. A region has a fixed size of 256m x 256m. In order to own mainland land a premium account is required, which incurs a monthly fee of US$9.95/month. In order to make as close a comparison as possible to land prices in Blue Mars, only the private island region prices will be used.

* It should be noted that the maximum allowed concurrent avatars is very difficult, if not impossible, to achieve in practice. Many residents of Second Life report that lag starts to become very noticeable when around 20 avatars are present in one region (20% of the maximum), and is similar to wading in mud when around 30 are present (30%), and few get to 40 avatars present (40%) without crashing.

In addition to the above prices, Linden Labs will apply Value Added Tax (VAT) for residents of the European Union.

Both Homestead regions and Openspace regions may only be owned by residents who currently own at least one Full Region.

Homestead regions have a lower performance than a Full Region and are intended for low-density rentals, quiet residential, or light commercial use.

Openspace regions are intended for very low-impact scenic use, such as ocean, forest, or countryside. No habitation, sub-rentals, or events are permitted.

Comparison
Making a direct comparison is not so straighforward, due to the different concepts involved, so I will provide an illustrative example.

A Blue Mars City is 2km x 2km of virtual land (4 sq km), and comes with a guaranteed support for 250 concurrent avatars.
The cost will be a setup fee of $2,000, and a monthly fee of $1,000.

To achieve the same with Second Life you will need initially a Full Region, plus 60 Openspace Regions. This will provide 61 x 256 x 256 = 3.99 sq km. You will get then get a maximum concurrent avatar capacity of 710, but no guaranteed lag-free support, although applying the practical lag-free limit of 20% the useable avatar concurrency is probably nearer 142.
The cost will be a setup fee of $16,500 and a monthly fee of $4,870.

Conclusion
Many people in the Second Life forums and over at SLU predicted high prices for Blue Mars land. I think the above example shows quite clearly that they got that wrong, big time. However, the Blue Mars prices are special pioneer rates for early adopters, and will likely be revised sometime in 2010, so I would recommend getting in early here, especially with the added bonus of up to 1GB of City Data to those who buy before the 1st February 2010, and with 2km x 2km of land starting from as low as $30/month (that is 62 times the amount of land you get with a SL region, and at almost 1/10 of the monthly price), now could never be a better time!